1. Field of the Invention
The invention relates to an apparatus and a method for easily and accurately analyzing volatile organic substances suspending or floating in the atmosphere, and more particularly to such an apparatus and a method preferably applicable to manufacturing steps of a semiconductor device such as an analysis of organic substances involved in an air current in a clean room.
2. Description of the Related Art
An analysis of organic substances is conducted, for instance, for observing a pollution level of an atmosphere. In such an analysis, as illustrated in FIG. 1, volatile organic substances involved in an atmosphere are first trapped in an adsorbent in step 1. FIG. 2 illustrates an example of an apparatus for trapping gaseous organic substances. The apparatus comprises a pump 2, a flow meter in fluid communication with the pump 2, an adsorbent 4 contained in a duct 4a in fluid communication with the pump 2, and a support 4b for maintaining the adsorbent 4 at a certain elevation. The atmosphere 1 such as an air in a clean room is evacuated by and into the pump 2 through the adsorbent 4 for a certain period of time. The flow meter 3 regulates a flow rate of the atmosphere 1 to be constant. Thus, a certain amount of volatile organic substances can be trapped in the adsorbent 4. Then, in step 2 in FIG. 1, the adsorbent 4 is heated so that the adsorbed organic substances remove out of the adsorbent 4, and subsequently the organic substances are injected into an analyzer. Specifically, as illustrated in FIG. 3, a sample tube 5 containing therein the adsorbent 4 adsorbing volatile organic substances is heated to thereby concentrate the organic substances in trapping section 19. Then, the organic substances are separated into individual substance in a separation section 6, and then mass of each organic substance is measured in a mass measurement section 7. A data processor 8 qualitatively and quantitatively analyze the organic substances based on data supplied from the separation section 6 and mass measurement section 7. A gas chromatographic mass spectrometery is used in the analysis.
Though the above mentioned method is directed to an analysis of volatile organic substances present in an atmosphere, the method is applicable, for instance, to an analysis of organic substances present in an air current in a clean room. Japanese Unexamined Public Disclosure No. 2-201159 has suggested a method for analyzing total organic carbon present in a gas so as to upgrade the gas which is to be used in a process for manufacturing a semiconductor device. Such a gas includes, for instance, inert carrier gas such as nitrogen gas, doping gas used for forming a diffusion layer, and oxygen gas used for forming an oxide layer. The total organic carbon analyzing method includes steps of adsorbing organic carbon present in a gas into an adsorbent in cooled and concentrated state, heating the organic carbon to thereby remove from the adsorbent, and measuring organic carbon with a total organic carbon spectrometer. The Disclosure states that in accordance with the method, organic substances present in a gas to be used for manufacturing a semiconductor device can be wholly measured as carbon dioxide gas regardless of kinds of organic substances.
The above mentioned two methods for analyzing organic substances are directed to an analysis of organic substances present in a gas such as an atmosphere and a gas to be used for manufacturing a semiconductor device. On the other hand, Japanese Unexamined Public Disclosure No. 2-262055 has suggested a method for analyzing organic substances adhered to a semiconductor substrate such as a wafer during a LSI manufacturing process. The method includes the steps of extracting organic substances adhered on a wafer with CO.sub.2 supercritical fluid, adsorbing the organic substances to an adsorbent in concentrated state, heating the wafer to thereby remove the organic substances out of the wafer, and analyzing the organic substances with a gas chromatography or a gas chromatographic mass spectrometery.
As illustrated in a flow chart of FIG. 1, the above mentioned three methods for analyzing organic substances commonly include the steps of trapping gaseous organic substances into an adsorbent, such as diatomaceous earth and tenax, which selectively adsorb gaseous organic substance, in concentrated state to thereby enhance sensitivity, heating the adsorbent to thereby remove the organic substances out of the adsorbent, and injecting the organic substances into an analyzer.
However, the above mentioned methods for analyzing organic substances have problems as follows.
An adsorbent, which is said to be able to selectively trap organic substance, has different trapping rates for each of organic substances, and hence it may be difficult for a certain adsorbent to trap a certain organic substance. Accordingly, it is required to prepare and use adsorbents each having different trapping rate. In addition, when an organic substance adsorbed into an adsorbent is to be removed out of the adsorbent by heating, an organic substance may be solved out from the adsorbent together with the adsorbed organic substances. Thus, it is quite difficult to analyze organic substances removed out of the adsorbent, if they were present in a quite small amount.
In a method to be applied to a semiconductor device manufacturing process, it is indispensable to identify organic substance which deleteriously affect a manufacture of a semiconductor device, among various organic substances present in an atmosphere. Thus, the above mentioned method and apparatus using a total organic carbon spectrometer is not suitable for a semiconductor device manufacturing process, because the method and apparatus do not identify organic substances and analyze organic substances which might include one deleteriously affecting a semiconductor device.
It has been recently understood that if a semiconductor substrate adsorbs organic substances present in an atmosphere, a certain kind of organic substance enormously degrades an insulative pressure proof of a silicon oxide layer. In addition, organic substances suspending or floating in an atmosphere has a low gasification temperature and a low temperature at which the organic substance is removed from a substrate. More specifically, these temperatures are equal to or lower than 400 degrees centigrade. Consequently, when a silicon oxide layer is to be grown on a semiconductor substrate on which organic substances stay as residue, the residual organic substances are decomposed and released into an atmosphere with the result that a silicon oxide layer may have a crack therein. Such a crack degrades semiconductor device properties, a yield of manufacturing semiconductor devices, and quality of semiconductor devices.
As having been described so far, with higher densification and integration of a semiconductor device, it is now necessary to measure organic substances with high sensitivity, which deleteriously affect the manufacture of semiconductor devices.
However, all of the above mentioned conventional apparatuses and methods for analyzing organic substances have problems that a certain organic substance is quite difficult to be adsorbed into a certain adsorbent, that when an adsorbent is heated for removing organic substances adsorbed therein out of the adsorbent, another organic substances are also solved out, resulting in that it is difficult to analyze a quite small amount of adsorbed organic substances, and that a total organic carbon spectrometer cannot identify organic substances which might deleteriously affect a semiconductor device.